Counter-terrorism
investigators have been investigating Khalid Masood’s life and
associations across Britain as they raced to discover what led him to
kill four people and injure more than 50 in Wednesday’s murderous
attack in Westminster.

A joint
investigation by Scotland Yard’s counter-terrorism command and MI5
led to new arrests on Thursday in London, Birmingham and Manchester
with two people still in custody, while properties were searched in
Wales and southern England.

Police want to
discover if Masood, who was shot dead by armed police, was
encouraged, supported or directed in the atrocity. The 52-year-old
mowed down people in the heart of London then stabbed a police
officer to death as he tried to burst into the Palace of Westminster.

The Guardian
understands detectives are scouring large amounts of CCTV camera
footage to see if there is any recording of Masood scouting the
Westminster area before the attack.

They are also using
camera records and automatic number plate recognition technology to
see if they show his car in the area, when Masood could have been
working out where the best place to run people over might have been
and what weak points there were in Palace of Westminster security in
the run-up to the attack. They will also repeat the same
time-consuming lines of inquiry to see if a possible accomplice may
have done the same.

Two minutes before
the attack started at about 2.40pm on Wednesday, Masood used the
WhatsApp communications app on his mobile phone, although it is not
known why.

On Friday police
named Masood’s fourth victim as 75-year-old Leslie Rhodes. Two
people remained in a critical condition in hospital.

MI5 is urgently
trying to discover how someone could come to believe in Islamic
State’s violent ideology and hatch a murderous plot against
high-profile targets to gain worldwide attention, all without the
domestic security service having any idea it was coming.

Fresh details about
Masood’s life have emerged. He had a 20-year history of offending
that saw him jailed at least twice. He had converted to Islam more
than a decade ago, and used several names during his life, having
been born as Adrian Elms and brought up as Adrian Ajao before
adopting a Muslim name.

So far, police
believe he acted alone when he drove a hired car into civilians on
Westminster Bridge before running into the grounds of parliament and
stabbing a police officer to death. It emerged that on the eve of the
attack he had checked into a Brighton hotel under his own name. The
hotel manager said that Masood had been “laughing and joking”.

Mark Rowley,
Britain’s top counter-terrorism officer, said: “Whilst there is
no evidence of further threats, you will understand our determination
to find out if he either acted totally alone inspired by terrorist
propaganda or if others have encouraged, supported or directed him.”

He described as
“significant” two arrests made in Manchester and Birmingham.

Since the attack,
police have arrested 11 people on suspicion of preparing terrorist
acts. On Friday they released eight without charge, having eliminated
them from their inquiries, and one woman was released on Thursday on
bail. The two in custody are a 58-year-old man and a man aged 27,
both arrested on Thursday in Birmingham.

An address searched
by police in Carmarthenshire, Wales, is believed to be a farmhouse,
belonging to his mother.

With key questions
unanswered about the man who inflicted the worst mass casualty
terrorist attack on Britain in more than a decade, Rowley said
investigators needed help from the public.

“We remain keen to
hear from anyone who knew Khalid Masood well; understands who his
associates were; and can provide information about places he has
recently visited,” he said.

“There might well
be people out there who did have concerns about Masood but weren’t
sure or didn’t feel comfortable for whatever reason in passing
information to us.”

The man described by
Isis propagandists as a “soldier” in their campaign of violence
was born Adrian Elms, and the Guardian has established he grew up in
Tunbridge Wells, Kent, with two half-brothers.

But in 2000 he was
jailed for grievous bodily harm after stabbing a man in the face
following a row that was reported at the time to have had racial
overtones. After being released from jail he attacked someone else
and was imprisoned again.

Seventeen people are
still being treated for their injuries in five London hospitals. Two
of those are in critical condition, according to the Metropolitan
police, with one person’s injuries considered life-threatening. One
police officer struck on the bridge, 35-year-old PC Kris Aves, is
feared to have suffered severe injuries.

More than 50 people,
of at least 12 nationalities, were injured in Masood’s attack on
Westminster Bridge. Of these, 31 initially required hospital
treatment.

One woman rescued
from the Thames, Andreea Cristea, 29, from Romania, was in London
with her fiance, who had been planning to propose to her. Masood’s
car struck her, throwing her into the river. She underwent an
emergency operation for a blood clot on the brain.

Prince Charles
visited some of the injured in hospital as police named the fourth
dead victim of Masood’s attack. Rhodes, of south London, died in
hospital overnight from his injuries sustained on Westminster Bridge.
The others killed in the attack were PC Keith Palmer, 48, who was
fatally stabbed by Masood inside the parliamentary gates; Kurt
Cochran, 54, an American tourist, and Aysha Frade, 43, a London
teacher. An internet fundraising appeal for Palmer’s family has
raised more than £600,000.

Past terrorist
attacks in Britain have led to debates about new laws or initiatives
to counter the lure that violent jihad has for a small number of
British based people.

Pent-up anger among
law enforcement and government has spilled over, with Downing Street
demanding social media companies do more to expunge extremist
material from the internet. The prime minister’s spokesman said
firms such as Facebook and Google “can and must do more” to
remove inflammatory material from the web and that it was up to them
to respond to public concern.

“Social media
companies have a responsibility when it comes to making sure this
material is not disseminated and we have been clear repeatedly that
we think that they can and must do more,” the spokesman told
journalists.

“We are always
talking with them on how to achieve that. The ball is now in their
court. We will see how they respond.”

In the aftermath of
the attack, hundreds of people were rushed into Westminster Abbey,
which was used as a sanctuary in case of further attacks. On Friday
the archbishop of Canterbury and Muslim leaders were among those who
joined an interfaith service to remember the victims of Masood.